Skip to main content

Full text of "Circuits"

See other formats

Laser Tripwire and Alarm Using Snap Circuits 


Laser Tripwire and Alarm Using 
Snap Circuits 

Written By: KRA5H 



• 100K ohm Resistor # 6SC R5 (2) 
1 0.02uF Capacitor # 6SC C1 (1) 
1 Speaker #6SCSP(1) 
1 Battery Holder (2-AA) # 6SC B1 (3) 
1 PNP Transistor # 6SC Q1 (1) 
1 NPN Transistor # 6SC Q2 (2) 
SCR #6SC 0-3(1) 
Pivot Stand Base # 6SC PSB (1) 
Pivot Post #6SCPSP(1) 
Pivot Top #6SCPSTm 
Photosensitive Resistor # 6SC RP (1) 

• Relay #6SC S3 (1) 
Single Snap Conductor # 6SC 01 (5) 
Conductor with 2-snaps # 6SC 02 (6) 

• Conductor with 3-snaps # 6SC 03 (4) 
Make Projects Page 1 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Conductor with 4-snaps # 6SC 04 (2) 
Conductor with 5-snaps # 6SC 05 (1) 
Conductor with 7-snaps # 6SC 07 (1) 
Jumper Wire 18" (Black) # 6SC J1 (1) 
Jumper Wire 18" (Red) # 6SC J2 (1) 
Jumper Wire 8" (White) # 6SC J3F (1) 
• Jumper Wire 4" (Blue) #6SCJ4(1) 
Laser pointer (1) has a green laser pointer similar to the one I use in this article. If you 
happen to have the Spynet Laser Tripwire laser you may use it as your laser trip wire. I 
used it in the demonstration video. It has two axes of rotation, horizontal and vertical, 
and is much easier to aim at the photoresistor. 
Magnetic Chip Clip (1) 

Serves the dual purpose of clamping the on/off switch in the "on" position and as a stand 
for the laser pointer-! got mine for free from an Indiana Fever game. 
Object(s) of your choosing to stack under the laser pointer to aim it at the photoresistor 


In this article I used a couple of packs of peanutbutter crackers. 


In this article you will learn how to build a laser tripwire and alarm out of Snap Circuits. You 
will build two circuits: the laser tripwire circuit and the alarm circuit. The alarm is a very 
simple two-transistor oscillator that is switched on by the relay in the laser tripwire circuit. 
Once you have built the two circuits, you will then learn how to set up the laser pointer 
where somone is likely to break the laser beam and trip the alarm. The Spynet Laser 
Tripwire was the inspiration for this build. I wanted to see if I could build my own laser 
tripwire. I also improved it by adding the relay to switch on an external circuit. I used the 
two-transistor oscillator as an example, but one could use the relay to switch on any number 
of external circuits such as floodlights, home alarm systems, etc. 

Actually, the laser tripwire was an interesting problem to try to solve. In a standard 
transistor-SCR (thyristor) circuit, you often see an LED where I put the relay. 

) Make Projects Page 2 of 1 5 

Laser Tripwire and Alarm Using Snap Circuits 

The SCR is advertised as a latch-on device-once switched on, it is supposed to stay on, but 
it don't work as advertised: 

If you replace the relay with an LED and break the laser beam (or other light source), that is, 
stop the light source from shining on the LDR (light dependent resistor) the LED switches on 
as advertised. When you restore the light shining on the LDR, however, the LED switches 
back off again. 

So, I decided to insert the relay to switch on another circuit that would stay on even when 
power was cut to the relay. Oddly enough, the relay somehow keeps the SCR switched on 
but I'm not sure why. 

In a single stroke (and by sheer accident!) I solved the problem and can now switch on any 
external circuit via the relay without having to add any additional components to make the 
externlal circuit remain on. And it will remain on until I switch the power off to the laser 
tripwire circuit. 

Wierd. But it may be useful to others working on any number of "electric eye" applications 

All Snap Circuits parts used here come from two sets: Snap Circuits Extreme 750, and Snap 
Circuits Green. Both sets are available at your local RadioShack store. Individual parts can 
be ordered from if needed 

) Make Projects Page 3 of 1 5 

Laser Tripwire and Alarm Using Snap Circuits 

Step 1 — Overview 

• Snap Circuits is an educational toy 
that teaches electronics with 
solderless snap-together electronic 
components. Each component has 
the schematic symbol and a label 
printed on its plastic case that is 
color coded for easy identification. 
They snap together with ordinary 
clothing snaps. 

• The components also snap onto a 
10X7 plastic base grid analogous 
to a solderless breadboard. 

• All the kits include manuals printed 
in color with easy-to-follow 
diagrams to assemble the projects. 
The illustrations for each project 
look almost exactly like what the 
components will look like on the 
base grid when finished. 

• Because the electronic symbol is 
printed on each electronic 
component, once the project is 
completed, it will look almost 
exactly like an electronic 

) Make Projects 

Page 4 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 2 — Assembling the alarm circuit. 

The alarm circuit is a very simple two-transistor oscillator circuit. 

) Make Projects 

Page 5 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 3 

• When the laser tripwire relay 
closes the alarm circuit (switches 
the alarm circuit on), here's what 

• Step 1 . The 0.02uF capacitor C1 
begins to charge up. Once the 
capacitor has enough charge, it will 
switch transistor Q2 on. 

• Step 2. When Q2 switches on, this 
causes transistor Q1 to switch on. 

• Step 3. When Q1 switches on, it 
supplies power to the speaker 
which makes a sound. 

• Step 4. Capacitor C1 begins to 
discharge to ground. 

• Step 5. Once the charge in the 
capacitior is low enough, this 
switches transistor Q2 off. 

• Step 6. When Q2 switches off, this 
will switch off transistor Q1. When 
Q1 switches off this cuts the power 
to the speaker and the speaker 
stops making a sound 

• Step 7. Go back to step 1 . This 
process repeats over and over 
again, ad infinitum, and you hear a 
high pitched tone until you cut the 
power to the circuit. 

) Make Projects 

Page 6 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 4 — Laser Tripwire part 1. 

• The photoresistor acts as a dark 
detector. As long as the laser beam 
shines on the photoresistor, this 
reduces its resistance. Since 
resistance in this part of the circuit 
is reduced, current can flow to the 
base of the transistor Q2 switching 
it on. 

• While Q2 is switched on, current 
will not flow to the SCR Q3. Once 
the laser beam is cut, that is, 
blocked from shining on the 
photoresistor, the photo resistor 
will increase its resistance and 
current will be cut from the base of 
the transistor Q2. When Q2 is 
switched off, current can then flow 
to the SCR Q3 switching it on. 

• When Q3 is switched on, this will 
power the electromagnet of the 

) Make Projects 

Page 7 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 5 — Laser Tripwire part 2. 

• You've probably seen a relay race 
where one runner hands off a baton 
to another runner. Similarly, an 
electronic relay hands off control 
from one circuit to another. A relay 
is a very simple device consisting 
of an electromagnet, an armature 
(a switch that closes when 
attracted by the electromagnet), 
and a spring that is connected to 
the armature. 

Step 6 — Laser Tripwire part 3. 

• When the SCR Q3 is switched on, 
current flows to the relay charging 
up the electromagnet. The 
electromagnet creates a magnetic 
field that attracts the armature to 
close the alarm circuit (switches 
the alarm circuit on), and you hear 
the high pitched tone created by 
the alarm circuit. 

) Make Projects 

Page 8 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 7 — Laser Tripwire part 4. 

• What's interesting about the SCR is 
that once it is switched on, it 
remains on providing current to the 
relay and the alarm sound will 
remain on until you turn the slide 
switch S1 off. 

) Make Projects 

Page 9 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

Step 8 — Laser Tripwire part 5. 

• After a person or object passes 
through the laser beam, the beam 
will shine on the photoresistor 
again reducing the resistance. 
Normally this would restore the 
current to the base of transistor 

• This would then switch Q2 on 
again, cutting the power to the SCR 
Q3 switching it off and thus cutting 
the current to the relay's 
electromagnet switching the alarm 
circuit off. 

• But, in the case of this circuit, the 
relay seems to be drawing enough 
power from the batteries to keep 
the SCR switched on and 
preventing the transistor from 
switching on again and cutting the 
power to the SCR. 

) Make Projects 

Page 10 of 15 

Laser Tripwire and Alarm Using Snap Circuits 
Step 9 

) Make Projects Page 11 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

) Make Projects 

• In the case of the green laser 
pointer, you have to hold the on/off 
button down to fire the laser and 
when you release the button, the 
laser switches off. So, you have to 
have a method of holding the laser 
pointer switch in the "on" position. 

• One easy method is to take a twist 
tie from a loaf of bread and wrap it 
around the laser pointer and the 
on/off button and then twist the tie 
tightly so that it holds the switch in 
the "on" position. I, on the other 
hand, use a different method to 
keep my green laser pointer 
switched on. 

• I, serendipitously, found that the 
Indiana Fever magnetic chip clip 
opens wide enough to insert my 
green laser pointer in its jaws. The 
chip clip also has a set of sawtooth 
teeth that catch on the barrel of the 
laser pointer holding it in position 
and preventing the clip from 
slipping off the barrel. 

• Also the chip clip clamps on the 
on/off switch and is strong enough 
to hold it in the "on" position. The 
clip can also be used as a stand to 
raise the laser pointer off the 
ground. You'll have to stack 
something underneath the laser 
pointer to aim it at the 
photoresistor-in my case I used a 
couple of packs of peanutbutter 

Page 12 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

• No worries if you don't live in 
Indianapolis nor attend professional 
or minor league sports events. You 
can probably find one of these chip 
clips being given away free with the 
appropriate corporate logo stamped 
on them at conventions, Earth Day 
celebrations, county/state fairs and 

) Make Projects Page 1 3 of 1 5 

Laser Tripwire and Alarm Using Snap Circuits 
Step 10 

) Make Projects Page 14 of 15 

Laser Tripwire and Alarm Using Snap Circuits 

• In this picture I used the Spynet 
Laser Tripwire laser. It is much 
easier to aim at the photoresistor 
since it has two axes of rotation- 
vertical and horizontal. 

• To set the laser tripwire ensure that 
the laser beam is set across a 
space where someone is likely to 
break the beam such as across a 
doorway or across a hall. 

• Recall too that that the laser dot 
from the pointer will project a fairly 
long distance so you could bounce 
it off a set of mirrors around the 
entire perimeter of your home so 
that anyone walking on to your 
property will break the beam. 

• For example, if your neighbor down 
the street lets his or her dog do its 
business on your lawn without 
cleaning up after it, you can catch 
him or her in the act. 

• Once you have the laser dot 
positioned on the photoresistor, 
turn the circuit on by moving the 
slide switch S1 to the "on" position. 
Pass your hand through the laser 
beam to test it. You will hear the 
relay click and the alarm sound. 
Switch the circuit off and back on 
again to reset it. 

• You can watch a video of the Laser 
Tripwire being demonstrated here . 

st generated on 2012-11-01 12:12:09 AM. 

) Make Projects 

Page 15 of 15